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a related question, is the distribution of FA values in a regions generally correlated with the amount of white matter and gray matter in that region? If so is there a particular range of FA values you would assign to gray matter? On Tue, Sep 27, 2011 at 11:26 AM, Marenco, Stefano (NIH/NIMH) [E] < [log in to unmask]> wrote: > Saad, what do you mean by a "secondary" fiber? Stefano Marenco > > -----Original Message----- > From: Saad Jbabdi [mailto:[log in to unmask]] > Sent: Tuesday, September 27, 2011 11:05 AM > To: [log in to unmask] > Subject: Re: [FSL] RE : [FSL] Correlation between FA and tractography > > Hi Hervé > Your explanation would only make sense if: > - You looked at FA within your tracts instead of the whole skeleton > - There were other cortical targets where the effect was in the > opposite direction (tracts that don't reach a target must go somewhere else) > > Note also that FA correlates (negatively) with dispersion (uncertainty) for > the primary fibre, but not necessarily for the secondary one. > > Saad > > > On 26 Sep 2011, at 10:50, LEMAITRE Hervé Université Paris Sud wrote: > > > Hi Matt and Saad, > > > > To clarify my point, we found that as the mean FA of the entire skeleton > increased, the number of samples decreased between an ROI in the corpus > callosum and cortical regions. We did this correlation for different > cortical regions (seeds_to_...) and it is almost systematic. > > One explanation was that the higher is your anisotropy the smaller is > your dispersion, and so, the lower is the number of streamlines that go from > one seed to the targets. Could you tell me if this is an > over-interpretation. > > Which kind of data would you see to have a good sense of what is going > on? > > > > Thanks again, > > > > Hervé > > > > ________________________________________ > > De : FSL - FMRIB's Software Library [[log in to unmask]] de la part de > Matt Glasser [[log in to unmask]] > > Date d'envoi : vendredi 23 septembre 2011 17:45 > > À : [log in to unmask] > > Objet : Re: [FSL] Correlation between FA and tractography > > > > To clarify, you found that as FA increased the number of streamlines > > decreased and vice versa? > > > > Increased amounts of crossing fibers could lead to reduced FA, but more > > connectedness between many regions. > > > > You would probably have to show more data for me to be able to better > > interpret your findings, however. > > > > Peace, > > > > Matt. > > > > -----Original Message----- > > From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On > Behalf > > Of Herve Lemaitre > > Sent: Friday, September 23, 2011 8:17 AM > > To: [log in to unmask] > > Subject: [FSL] Correlation between FA and tractography > > > > Dear FSL users, > > > > we ran tbss on several subjects and also bedpostx/probtrackx to do > > tractography between one part of the corpus callosum and different > regions > > of the cortex. > > Computing the average number of samples between the seed and each > cortical > > region, we found a negative correlation between global mean skeleton FA > and > > number of samples for each cortical region. > > Is it normal to have this kind of negative correlation and how can we > > explain it? > > > > Thanks, > > > > > > Hervé Lemaître > > > > -- > Saad Jbabdi > University of Oxford, FMRIB Centre > > JR Hospital, Headington, OX3 9DU, UK > (+44)1865-222466 (fax 717) > www.fmrib.ox.ac.uk/~saad > -- --Saeideh